This application relates generally to gas turbine engine rotor assemblies and, more particularly, to bearing assemblies for gas turbine engine rotor assemblies.
Gas turbine engines typically include a rotor assembly, a compressor, and a turbine. The rotor assembly includes a fan that includes an array of fan blades extending radially outward from a rotor shaft. The rotor shaft transfers power and rotary motion from the turbine to the compressor and the fan and is supported longitudinally with a plurality of bearing assemblies. Additionally, the rotor assembly has an axis of rotation that passes through a rotor assembly center of gravity. Known bearing assemblies include rolling elements and a paired race, wherein the rolling elements are supported within the paired race. To maintain rotor critical speed margin, the rotor assembly is supported on three bearing assemblies, one of which is a thrust bearing assembly and two which are roller bearing assemblies. The thrust bearing assembly supports the rotor shaft and minimizes axial and radial movement of the rotor shaft assembly. The remaining roller bearing assemblies support radial movement of the rotor shaft.
During operation of the engine, a fragment of a fan blade may become separated from the remainder of the blade. Accordingly, a substantial rotary unbalance load may be created within the damaged fan and carried substantially by the fan shaft bearings, the fan bearing supports, and the fan support frames.
To minimize the effects of potentially damaging abnormal imbalance loads, known engines include support components for the fan rotor support system that are sized to provide additional strength for the fan support system. However, increasing the strength of the support components undesirably increases an overall weight of the engine and decreases an overall efficiency of the engine when the engine is operated without substantial rotor imbalances.
Other known engines include a bearing support that includes a mechanically weakened section, or primary fuse, that decouples the fan rotor from the fan support system. During such events, the fan shaft seeks a new center of rotation that approximates that of its unbalanced center for gravity. This fuse section, in combination with a rotor clearance allowance, is referred to as a load reduction device, or LRD. The LRD reduces the rotating dynamic loads to the fan support system.
After the primary fuse fails, the pitching fan rotor often induces a large moment to a next closest bearing. The next closest bearing is known as the number two bearing position. The moment induced to the number two bearing induces high bending and stress loads to the fan rotor locally. To relieve the high bending stresses, the radial and pitching rotation stiffness of the number two bearing position are often softened or released. However, in order to maintain a safe shutdown and subsequent windmill of the engine during the time it takes to land an aircraft, the remaining bearing assemblies must remain functional and maintain radial stiffness to provide fan critical speed margin above a fly home windmilling speed.
In an exemplary embodiment, a rotor assembly for a gas turbine engine includes a rotor assembly and support system that reduce dynamic loads to the overall engine structure. The rotor assembly and support system include a rotor shaft coupled to a fan and supported longitudinally with a plurality of bearing assemblies and supports. Specifically, a first bearing housing includes a primary fuse that fails when exposed to a pre-determined load as a result of a fan imbalance. A second bearing assembly aft of the first bearing assembly and serially downstream from the first bearing assembly further reduces dynamic loading to the support frame and thus, facilitates reducing static shaft bending stresses induced locally to the bearing.
The second bearing assembly includes a paired race, a rolling element, and a mounting race. The paired race includes an inner race and an outer race, each sized to receive the rolling element therein. The mounting race includes a spherical face and is secured to the bearing assembly with retainers. The retainers fail when exposed to a pre-determined moment load, but withstand normal engine operating loads. These retainers are hereinafter referred to as secondary fuses.
During operation, after the primary fuse fails and a moment load above a pre-determined level is transmitted to the second bearing assembly, the second bearing assembly retainer fails. After the secondary fuse failure, the moment stiffness of the number two bearing is released, allowing further pitch rotation of the fan shaft on the spherical face. As a result, the bearing assembly facilitates reducing static bending loads to the rotor and dynamic loads transmitted to the support frame structure. Radial support of the bearing position is then maintained providing critical speed margin over fan windmilling speeds.